1. Field of the Invention
The present invention relates to electromagnetic rotary motion devices and has its general object to provide a device in which a movable member is electromagnetically driven in a circular path by a torque which is proportional or otherwise related to an electric control signal applied to the device.
2. Review of The Prior Art
The prior art diaphragm-assisted automatic exposure control device, incorporated in a cinema motion camera, for example, commonly uses a fixed permanent magnet and a coil which is rotatable thereon with a shaft drivingly connected to the diaphragm of the camera. The diaphragm is mechanically biased to a position to form a fully open aperture defined by diaphragm blades and is driven by the rotation of the coil on the permanent magnet so that the working area of the aperture in the diaphragm is varied by the angular displacement of the coil. The coil is connected to an external control circuit including photoelectrical transducer elements to produce an electrical signal which is representative of the brightness of an object to be photographed. The coil is thus energized with a current which is related to the brightness of the object so that the aperture in the diaphragm is set to be optimum for the particular brightness of the object. Numerous drawbacks have been pointed out as inherent in the exposure control device of this nature, including the insufficient density of the magnetic fluxes acting on the coil and the limited allowance for the angular displacement of the coil on the permanent magnet.
To cope with the limited angular displacement of the coil, for example, it has been an ordinary practice to have the diaphragm blade driven at a point considerably remote from the principal axis of a lens system of the camera for amplifying the angular displacement of the diaphragm blade. This results in unstable and unbalanced movement of the diaphragm blades and in a tendency of an excessive braking torque being applied to the blades when the blades are being brought to a standstill. Difficulties are, moreover, encountered in totally enclosing the exposure control device of the above described construction, so that the coil on the permanent magnet is subject to an influence of an external magnetic field, if any, and to an ingress of dust and moisture which are responsible for the deterioration of the performance of the exposure control device.
The image pick-up tube of the television camera or video telephone, on the other hand, uses a faceplate of a light sensitive film which is susceptible to mechanical shocks and contact so that the diaphragm should be operable not only to control the quantity of light to be admitted to the tube but to protect the faceplate from such mechanical shocks and contact. Thus, the diaphragm-assisted exposure control device for use in the television camera or video telephone commonly uses a servo motor so that the rotational motion achieved in the motor is carried transmitted to the diaphragm. By reason of the particular construction and principle of operation of the servo motor, however, it is practically impossible to have the rotational motion of the motor directly transmitted to the diaphragm without aid of reduction gears or any other mechanical power transmission means.
The provision of such mechanical transmission means, and also of a considerably intricate electrical control arrangement which is usually required for the servo motor, has been not only a major impediment to reducing the overall dimensions of the exposure control device but a cause of objectionable friction and play which are responsible for the deficient response of the exposure control device to an input electrical signal. Another drawback inherent in the exposure control device using the servo motor is that the servo motor has an axis of rotation which is not aligned with a principal axis of the lens system of the television camera or video telephone so that unbalanced and unstable movement of the diaphragm blades result, similarly to the exposure control devices for the still or cinema motion cameras. Restrictions are, moreover, met in achieving sufficient magnetic fluxes due to the particular configuration of the exposure control device using the servo motor so that only a limited amount of driving torque is available to the servo motor.